The leading edges of airfoil blades such as helicopter main and tail rotor blades are subject to erosion from contact with airborne matter such as rain or sand. In desert environments, sand erosion is often experienced on airfoil blades. The blade leading edge is typically protected by a metallic erosion strip consisting of nickel (Ni) over titanium (Ti) on the outboard portion of the blade and titanium on the inboard portion. Exemplary Ti-based substrate alloys include Ti-6Al-4V, Ti-8Al-1Mo-1V, Ti-6Al-2Sn-4Mo-2Zr, Ti-6Al-2Sn-4Mo-6Zr, and Ti-5.5Al-3.5Sn-3Zr-1Nb. Other substrates include, but are not limited to, stainless steels (e.g., 17-4PH) and Ni-base superalloys (e.g., alloy 718).
Sand, which is primarily made up of quartz, is significantly harder than both Ni and Ti. This hardness difference results in significant degradation of rotor blades in desert environments. As a result, this has become one of the largest logistics and maintenance burdens for helicopter operators in a sandy environment. Another significant problem is that a corona or halo, which is visible through night vision goggles, is generated around the rotor blades at night due to sand particles impacting the Ti leading edge. The erosion phenomenon has been widely studied, for example, by S. M. Wiederhorn, B. J. Hockey, Effect of material parameters on the erosion resistance of brittle materials, J. Mater. Sci. 18 (1983) 766-780; I. M. Hutchings, R. E. Winter, Particle erosion of ductile metals: a mechanism of material removal, Wear 27 (1974) 121-128; L. Zhao, M. Maurer, F. Fischer, E. Lugscheider, Surf. Coat. Technol. 185, (2004) 160-165; I. Finnie, Erosion of surfaces by solid particles, Wear 3 (1960) 87-103; J. G. A. Bitter, A study of erosion phenomena. Part I &2 Wear 6 (1963) 5-21 and 169-190; and I. M. Hutchings, Ductile-brittle transitions and wear maps for the erosion and abrasion of brittle materials, J. Phys. D: Appl. Phys. 25 (1992), A212-A221.
Typical metal erosion strips generally cover approximately one inch of the blade surface measured from the leading edge. The most severe wear in sand erosion patterns on, for example, a UH-60L main rotor blade, is generally at the tip cap and covers nearly 50% of the tip cap surface. Leading edge wear ranges from 6.0″ to 0.5″ in width measured from the end of erosion strip. The erosion takes place on the top and bottom surfaces of the blade as well as at the leading edge. The tail rotor blade erodes primarily at the tips with approximately one-half the length of the blade along the leading edge affected.
Recently, soft polymeric coatings in tape and sprayed forms have been applied onto blade leading edges to control sand erosion. These coatings provide improved service life when compared to uncoated substrates but require frequent repair and replacement. One polymeric coating evaluated in a test program has demonstrated a significant 4-5 times improvement of sand erosion resistance. However, polymeric coatings generally exhibit poor erosion performance in a rain environment due to their low strength and the high dynamic stress generated by the impacting raindrop. Erosion life of the polymer coating can be further reduced in extreme temperature and high humidity environments. The degradation from rain erosion is much faster if the rotor blade leading edge has been eroded by sand and small rock particles first, which causes surface defects that grow under the dynamic stress from the flow of raindrops upon impact.
In view of the above, there is a need to develop alternative erosion protection coatings.